Genomic damage, if left unrepaired, can lead to malignant transformation, or cell death by senescence (aging), necrosis or apoptosis. Among the variables that can affect the ultimate biological consequence of DNA damage to a particular cell are (i) the amount, type, and location of the DNA damage and (ii) the efficiency and bioavailability of the cellular DNA repair mechanism.
The activation of poly(ADP-ribose) polymerase (PARP) by DNA strand breaks is often one of the first cellular responses to DNA damage. PARP catalyzes the conversion of nicotinamide adenine dinucleotide (NAD) to multi-branched polymers containing up to 200 ADP-ribose residues. Increases in polymer levels of more than 100-fold may occur within minutes of DNA damage. Once synthesized, polymers are rapidly turned over, being converted to free ADP-ribose by the action of poly(ADP-ribose) glycohydrolase (PARG) (1). An ADP-ribosyl protein lyase has been proposed to catalyze removal of protein-proximal ADP-ribose monomers (2). FIG. 1 illustrates these processes schematically.
The process of activating PARP upon DNA damage can rapidly lead to energy depletion because each ADP-ribose unit transferred by PARP consumes one molecule of NAD, which in turn, requires six molecules of ATP to regenerate NAD. Additionally, NAD is a key carrier of electrons needed to generate ATP via electron transport and oxidative phosphorylation or by glycolysis. The overactivation of PARP due to substantial DNA damage can significantly deplete the cellular pools of NAD and ATP (3). ADP-ribose polymer metabolism, and thus PARP and PARG have been linked to the enhancement of DNA repair (4), limitation of malignant transformation (5), enhancement of necrotic cell death (6), and involvement in programmed cell death (7). To date, studies of the structure and function of the enzymes of ADP-ribose polymer metabolism have been mainly limited to PARP (8). Little is known about the function and regulation of PARG.